Since the equipment such as personal computers and the like using recording media has been made smaller in size and larger in capacity, making the spindle motors for driving recording media incorporated into the equipment smaller in size and larger in capacity is also demanded. Along with this, it is further required to also make the bearings of the spindle motors smaller in size and higher in accuracy.
Many ball bearings have hitherto been adopted as bearings used for the spindle motors. However, as making spindle motors smaller in size, particularly smaller in outer diameter, proceeds, use of the ball bearings of smaller outer diameters corresponding thereto causes the inner and outer races to be easily deformed as the motors are assembled, whereby realization of a sufficient accuracy of rotation is apt to be difficult in practice. Moreover, problems of noise and vibration are also inclined to occur.
In the case of the spindle motors for driving recording media, the high speed rotation accompanied by the smaller outer diameter thereof is required, so, the above-mentioned problems are further promoted. In addition, the accuracy of finishing the ball bearings has a certain limitation in spite of whether the outer diameters are large or small, and the case where the specification of the requirements is not satisfied is forecasted. For that reason, for example, JPA 149410 (1991) discloses a spindle motor adopting a hydrodynamic bearing construction, which is so constructed that a generally smaller sized (for example, a rotor hub having an outer diameter of approximately 40 mm or less) spindle motor is provided with a rotary sleeve at the inner peripheral side of the base of a substantially bowl-shaped rotor hub, and the rotary sleeve is fitted onto and rotatably supported by the outer periphery of a stationary support at the stator side, the hydrodynamic bearing construction being such that on the surfaces of the rotary sleeve and stationary support which come into sliding contact with each other, one or more grooves of predetermined shape are provided and filled with a lubricant.
However, in the case where such a hydrodynamic bearing construction is adopted, making the bearing parts smaller in size can be achieved more than making a ball bearing smaller thereby, leakage of a lubricant becomes a problem. When the lubricant leaks out of the motor, for example, it stains recording media or the like fitted onto and fixed to the rotor hub; therefore, such a staining must be avoided. In order to surely prevent leakage of the lubricant, use of a magnetic fluid seal is preferable and such an example is disclosed, for example, in JPA 60355 (19991). However, with such a magnetic fluid seal, it is considerably difficult, for example, to pour the magnetic fluid into the vicinity of the hydrodynamic radial bearing positioned at the middle portion of the interior of the rotor hub. In order to avoid such a problem, if an attempt is made to provide the hydrodynamic bearing on the outer periphery portion of the rotor hub. However, in this case, a large quantity of magnetic fluid is necessary, thereby bringing about an increase in cost.